Post-tensioned, below-grade concrete foundation system

ABSTRACT

A below grade, post-tensioned concrete foundation system for use in the construction of residential dwellings and other structures is described. The foundation includes a post-tensioned concrete floor slab formed on the soil base of a below grade excavation. Concrete foundation walls are formed on the perimeter of the post-tensioned floor slab. The weight of the superstructure of dwelling as well as the weight of foundation itself is distributed across the post-tensioned floor slab, thereby resisting the detrimental effects of expansive soils on the foundation.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to the following related applications, each of which is hereby incorporated in its entirety by reference as though fully set forth herein: U.S. provisional application No. 60/390,738 filed Jun. 21, 2002, entitled “Post-Tensioned Concrete Foundation System;” U.S. provisional application No. 60/410,532 filed Sep. 13, 2002, entitled “Post-Tensioned Below Grade Concrete Foundation System;” and U.S. provisional application No. 60/410,533 filed Sep. 13, 2002, entitled “Engineering Process for Post-Tensioned Below Grade Concrete Foundation System.”

BACKGROUND OF THE INVENTION

[0002] Expansive soils are a significant problem for the construction industry. This is especially true in the area of residential home construction. Over time, expansive soils can increase in volume under a home causing the foundation to crack and compromise the structural integrity of the home. Further, soil expansion can cause concrete slab floors to buckle and crack, which may damage finished interior flooring, for example, tile or hardwood. Fixing the problems created in foundations and concrete slabs by expansive soils can be extremely expensive and difficult to implement. The problems associated with expansive soils adversely affect both the homeowner and the residential construction trade.

[0003] In order to circumvent the problems caused by expansive soils, homebuilders have employed various building techniques to mitigate potential damage from expanding soil underneath a home. One such solution, shown in FIG. 1 as a general representation of exemplary prior art, is to support the foundation on concrete piers or caissons. FIG. 1 depicts a portion of a house foundation 6 supported by a concrete caisson 4. The house of FIG. 1 is modeled to include a basement 7 excavated below grade 9. However, the foundation 6 could likewise be a foundation on grade 9 without a basement and similarly be supported on caissons 4. The caisson 4 preferably rests upon bedrock 2 (or other suitable support) in order to provide a stable support for the foundation 6. The foundation 6 may be in the form of a concrete perimeter beam or footer at the level of the excavation floor 10. A concrete foundation wall 12 is formed on top of the foundation 6 and extends from the excavation floor 10 to above grade 9.

[0004] A caisson 4 is constructed by drilling a shaft in the ground, for example, until bedrock 2 is reached. The shaft may be anywhere between 10 and 30 feet deep or more. The concrete caisson 4 is then formed to rest upon the bedrock by pouring a cement mixture in the shaft. Multiple concrete caissons are put in place at a single home site in order to support the foundation 6. Again, the caisson 4 may support a foundation 6 of several configurations, for example, for a house with a basement 7, or a house built on grade 9.

[0005] One known system for withstanding the forces of expansive soils for houses built on grade is a post-tensioned concrete slab. Such post-tensioned concrete slabs act as both the foundation and the floor of the house. Steel tendons or cables are placed both longitudinally and laterally within the forms for the concrete slab. An additional cable may also be run around the perimeter of the foundation form. One end of each cable is anchored and the other end is stretched after the concrete slab has at least partially cured, placing the cable under tension and compressing the concrete slab, thereby increasing its tensile and bending strength under load. In some configurations, shallow, post-tensioned concrete beams are formed underneath the slab to provide further support. Because the post-tensioned concrete slab is formed directly on grade, while unitary integrity may be maintained, the slab, and therefore the entire house, may still move laterally on the soil surface as the soil expands and contracts. One way to counteract such movement is to support these slab-on-grade foundations by piers or caissons as previously described.

[0006] Understandably the cost and time involved in building a house increases significantly with the use of caissons. Cost increases are found in the expense of hiring a drilling rig plus the concrete forming the caissons. Further, additional time must be factored into the building process to allow for the drilling of the caisson shafts.

[0007] A construction design resistant to expansive soil effects generally used in areas where homes are built with basements is to substitute a structural floor 15, as shown in FIG. 1, for a concrete slab on the excavated floor 10 of the basement. Caissons 4, as previously described, may be incorporated in this design to provide additional support for the foundation 6. Once the foundation 6 and foundation walls 12 are formed, hangers 8 are attached above the excavated floor 10 along the foundation walls 12, and the desired structural system 15, e.g., wood, steel, or concrete, is installed to span the space between the foundation walls 12 to support the basement floor 16. In this design, a void or crawl space 14 remains between the excavated floor 10 at the bottom of the basement and the basement floor 16 suspended a few feet above. Because the structural basement floor 16 is spaced above the soil of the basement excavation, the soil can expand and contract without affecting the basement floor 16.

[0008] One of the downsides to using a structural basement floor 15 to combat expansive soils is increased costs and time in the building process. Creating the void under the structural basement floor 15 necessitates a greater basement excavation depth. This means there is greater time and cost in the excavation portion of construction. Also, in order to provide a standard eight-foot ceiling in the basement, for example, the concrete basement foundation walls 12 need to be higher than normal, perhaps as high as 11 feet, to provide the void 14 under the structural basement floor 15. Obviously, the additional concrete used in the foundation walls 12 result in a greater cost to the construction project. Another problem associated with this building technique is that the crawl space 14 under the structural basement floor 16 may promote an unhealthy environment within the home. The dank, dark void area 14 between the excavation floor 10 and the structural basement floor 16 is a conducive environment for vermin, spiders, and snakes. Further, it has been found that molds can flourish in this environment, which can create significant allergy and respiratory problems for the occupants of the home.

[0009] In view of the various drawbacks to the present construction designs and techniques for ameliorating the effects of expansive soils in residential construction, an alternative system for construction of homes with basements or other below-grade foundations would be desirable. The present invention addresses this issue.

SUMMARY OF INVENTION

[0010] The present invention seeks to combat the problem of expansive soils and avoid several disadvantages of the prior structures used to address expansive soil issues as discussed above. The system of the present invention provides a post-tensioned concrete floor slab formed below grade in the basement excavation of a home. Post-tensioning the floor slab provides compression of the floor slab in both planar directions (i.e., longitudinal and lateral directions). This compression significantly increases the tensile and bending or beam strength of the floor slab, providing greater resistance against the forces of expansion of the soil. The foundation walls, rather being supported on footings or caissons, as is the common practice in home construction, are supported directly by the perimeter edge of the top surface of the floor slab (hereinafter referred to merely as the perimeter edge). In this manner, the entire weight of the house sits upon and is distributed across the post-tensioned floor slab. Because the floor slab is post-tensioned, it can support the weight of the concrete walls in addition to the entire weight of the house. The post-tensioning of the concrete floor slab further distributes the weight of the house across the entire concrete floor slab. The combination of the post-tensioning of the floor slab and the increased downward force due to the weight of the house helps counteract the force of the expanding soil and significantly mitigates the typical effects of expanding soil on a foundation. With this design, the house will move as an integral unit. Because the house moves as an integral unit, the soil expansion must be sufficient to move the weight of the entire house. As a result, the house is more likely to remain unaffected by slight soil expansion forces that may have previously caused cracks to appear in the floor or in the foundation. Generally, the expansive soil force must be great enough to overcome the weight of the entire structure, which is distributed over the entire foundation floor. If the force of the expansive soil overcomes the weight of the entire structure, then the structure will likely react as a single unit, and the foundation and may shift slightly. This invention reduces the likelihood of structural damage normally caused by expansive soils, for example, cracking floors and walls.

[0011] Several aspects to the invention can better be understood by reference to different practical embodiments. A first embodiment is a house with a basement excavation at a single level. A second embodiment is detailed with respect to a house built with a structural garage floor. A third embodiment is discussed with respect to a foundation built upon multiple excavation levels. A fourth embodiment concerns the integration of counterforts along the basement walls in order to resist the lateral forces of the soil below grade on the basement walls.

[0012] A first embodiment of a residential dwelling with a basement may includes of a post-tensioned concrete slab below grade and an exterior concrete basement wall located on a perimeter edge of the post-tensioned concrete slab. This and each of the other embodiments described herein fulfills one of the advantages of the invention to resist the stresses on the foundation caused by expansive soils. The foundation may be further include post-tensioned concrete beams running underneath the concrete slab. The post-tensioned concrete slabs and beams may be formed by including tendons in their structures that are stressed to compress the concrete. In some embodiments a tendon may extend within the both the concrete slab and the beam, the concrete slab and a concrete basement wall, or all three.

[0013] One method of constructing a such a foundation begins with the excavation of the soil at a site below grade level. A plurality of tendons may be placed upon the floor of the excavation. A cement mixture is then poured over the plurality of tendons on the level below grade. The cement is at least partially cured to form a concrete slab. The concrete slab is then post-tensioned by applying tensile forces to the plurality of tendons. Once the concrete slab is post-tensioned, the exterior concrete walls of the foundation are formed on a perimeter of the concrete slab.

[0014] In a second embodiment of the foundation mentioned above, a high load bearing floor (e.g., a garage with a structural floor) is supported by a plurality of beams spanning a first exterior concrete foundation wall and a second exterior concrete foundation wall. The beams may also, or alternatively, span an interior concrete wall and an exterior concrete foundation wall.

[0015] A third embodiment of a residential dwelling according to the present invention may be constructed with a multi-level foundation composed initially of a first concrete slab, an exterior concrete basement wall on at least a portion of a perimeter edge of the first concrete slab, and a means for post-tensioning the first concrete slab. The multi-level dwelling further has a second post-tensioned concrete slab adjacent to the exterior concrete basement wall. At least one of the levels is below grade. At least one tendon is used to structurally integrate the two levels. A first portion of the tendon extends within the second post-tensioned concrete slab, and a second portion of the tendon extends within the exterior concrete basement wall and may extend within the first concrete slab as well.

[0016] A fourth embodiment of the invention concerns the addition of counterforts to the foundation. The novel counterfort is formed by an extension of a post-tensioned concrete slab in a basement excavation beyond a wall of a foundation supported by the post-tensioned concrete slab. A concrete member generally normal to the extension of the post-tensioned concrete slab is supported by the extension of the post-tensioned slab.

[0017] Other features, utilities and advantages of various embodiments of the invention will be apparent from the following more particular description of embodiments of the invention as illustrated in the accompanying drawings and defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is an elevation view in cross-section of prior art construction utilizing caissons and suspended structural floors.

[0019]FIG. 2 is a plan view of a post-tensioned, below grade, foundation for a home according to a first embodiment of the present invention.

[0020]FIG. 3A is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 detailing the dead end of a tendon in position in the post-tensioned concrete slab.

[0021]FIG. 3B is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 detailing the live end of a tendon in position in the post-tensioned concrete slab.

[0022]FIG. 3C is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 depicting the placement of a tendon within the concrete slab.

[0023]FIG. 4A is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 detailing the concrete slab and beam supporting an exterior basement wall.

[0024]FIG. 4B is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 detailing tendon placement in the concrete slab, beam, and exterior basement wall.

[0025]FIG. 5 is a plan view of a post-tensioned, below grade foundation supporting a structural floor garage according to a second embodiment of the invention.

[0026]FIG. 6A is an elevation view in cross-section of a detail of the structural garage floor of FIG. 5 attached to a basement wall.

[0027]FIG. 6B is an elevation view in cross-section of a portion of the structural garage floor of FIG. 5 supported by the basement wall and meeting with a driveway slab.

[0028]FIG. 7 is a plan view of a multi-level, below grade, post-tensioned foundation for a home according to a third embodiment of the present invention.

[0029]FIG. 8A is an elevation in cross-section of the multi-level interface of the post-tensioned foundation of FIG. 7 detailing the cantilever of the upper level of the foundation.

[0030]FIG. 8B is a partial elevation view in cross-section of the upper level slab, beam, and sill of the post-tensioned foundation of FIG. 2.

[0031]FIG. 9A is an elevation view in cross-section of a portion of the multi-level, post-tensioned foundation of FIG. 7 detailing a counterfort poured on top of a projection of a post-tensioned slab and beam.

[0032]FIG. 9B is a plan view of the counterfort of FIG. 9A.

[0033]FIG. 10 is a plan view of an arcuate window well counterfort placed on a projection of a post-tensioned slab.

[0034]FIG. 11A is a plan view of a window well counterfort housing an opening for a sump pump placed on a projection of a post-tensioned slab.

[0035]FIG. 11B is an elevation view in cross-section of the window well counterfort of FIG. 10A.

[0036]FIG. 12 is a fragmented elevation view in cross-section of a side wall of the post-tensioned foundation of FIG. 7 detailing an integrated wall and beam structure.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention is directed to a novel foundation structure for a residential dwelling with a basement or wherein the foundation of the house or dwelling is otherwise built in an excavation below grade. While the examples of the invention are described herein for use in residential dwellings, it is contemplated that the invention could also be implemented in commercial or municipal buildings, or any other structure amenable to these inventive techniques. The inventive foundation disclosed herein is designed to be substantially resistant to the forces of expansive soil that may act upon the foundation. At its base, the foundation structure of the present invention, for example, as depicted herein in FIGS. 1-4B, is a post-tensioned concrete floor slab formed upon the soil bottom of a basement excavation for the house. Exterior concrete foundation walls are built directly upon the perimeter of the post-tensioned concrete floor slab. In one embodiment, for example, as shown in FIG. 12, the foundation walls are structurally integrated with the post-tensioned floor slab by running portions of tensioning cables used to post-tension the floor slab both through the floor slab and upward into the foundation walls as well. In this manner the floor slab and exterior walls operate together as load supporting beams to support the load of the dwelling. Interior concrete basement walls or buttresses may likewise be formed on and structurally integrated with the basement floor slab with tensioning cables to further support interior loads of the home. Because the foundation is formed below grade, there is little problem of lateral movement of the foundation as found with post-tensioned slabs on grade.

[0038] Additional structural features can be combined with the basement post-tensioned floor slab foundation structure to either create more versatile home designs, further reinforce the strength of the foundation, or both. Because of the strength of the post-tensioned floor slab design, the dwelling can support substantial loads that might otherwise not be possible with regular home construction techniques. For example, in one embodiment as shown in FIGS. 5, 6A, and 6B herein, a structural garage floor is provided wherein the garage may be built over the basement. The significant weight of a car in the garage is transferred to and supported by the foundation. Additionally, because of this added weight providing a downward force on the foundation, the foundation is more resistant to the upward forces of underlying expansive soils.

[0039] Another possible structural variation provides for multi-level foundations. For example, rather than a structural garage as described above, a garage floor built on grade may be structurally integrated with the post-tensioned basement foundation as shown in FIG. 7. The garage floor slab may itself be post-tensioned to create an integral unit. Then the garage floor slab may further be integrated with the house foundation by training tensioning cables within both the garage floor slab and the basement floor slab and extending through a system of buttresses and counterforts. The garage floor slab may be viewed as cantilevered off of the adjacent foundation wall. With this design, the house foundation and the garage floor slab will respond to soil expansion as a unitary structure, thereby increasing the expansive soil force necessary to affect the structure, and also reducing the stresses on the superstructures of the house and garage, particularly at the intersection of each. Using the structural concept exemplified by the cantilevered garage, a foundation may be designed integrate post-tensioned basement floor slabs (and foundation walls) formed at multiple excavation levels into an integral structural unit. By transferring the load of the basement walls and the superstructure of the house to the post-tensioned floor slab, the same resistance to expansive soil problems is achieved.

[0040] With this design, the house will move as an integral unit. Because the house moves as an integral unit, the soil expansion must be sufficient to move the weight of the entire house. As a result, the house is more likely to remain unaffected by slight soil expansion forces that may have previously caused cracks to appear in the floor or in the foundation. Generally, the expansive soil force must be great enough to overcome the weight of the entire structure, which is distributed over the entire foundation floor. If the force of the expansive soil overcomes the weight of the entire structure, then the structure will likely react as a single unit, and the foundation and may shift slightly. This invention reduces the likelihood of structural damage normally caused by expansive soils, for example, cracking floors and walls.

[0041]FIGS. 2, 3A, 3B, 3C, 4A and 4B depict a first embodiment of a below-grade, post-tensioned foundation according to the present invention. The post-tensioned foundation 17 of FIG. 2 is shown in plan view and details the locations of various structural components of the foundation 17. The post-tensioned, concrete, basement floor slab 20 rests upon soil 19 at the bottom of the basement excavation as shown in greater detail in FIG. 3C. Concrete beams 18 (indicated by the thick dashed line in FIG. 2), which may be integral with the basement floor slab (e.g., formed monolithically), extend beneath the post-tensioned basement floor slab 20. The width and depth of the concrete beams 18 is determined according to engineering requirements based upon a number of factors, for example, the expansiveness of the soil and the load to be supported by the foundation. The concrete beams 18 are located around the perimeter edge 56 of the post-tensioned basement floor slab 20 and are also placed in positions across both the length and the width of the post-tensioned basement floor slab 20. The concrete beams 18 provide added structural support to the post-tensioned basement floor slab 20 (if needed) as will be described in greater detail later herein. The width of the concrete beams 18 may be expanded in certain locations to act as footers 21 to better support point loads bearing down from the superstructure of the house, for example, posts supporting a load bearing beam. In other embodiments, the post-tensioned, concrete basement floor slab 20 may be engineered to a sufficient thickness with appropriate tendon 22 placement and other reinforcement that the addition of beams 18 to the design is unnecessary.

[0042] Tensioning cables or tendons 22 (indicated by the thinner dashed lines of FIG. 2) are placed within the post-tensioned, concrete basement floor slab 20 running across both the length and width of the post-tensioned floor slab 20. In this manner an orthogonal web of tendons 22 is formed within the post-tensioned floor slab 20. In alternative embodiments the tendons 22 may run in only one direction or at varying angels according to engineering specifications. In a location where a concrete beam 18 extends beneath the post-tensioned floor slab 20, a tendon 22 may undulate such that portions of the tendon 22 are within the area defined by the beam 18 and portions of the tendon 22 are within an area defined by the post-tensioned floor slab 20. Alternatively, or additionally, depending upon the engineering requirements, separate tendons 22 may each run solely within the beam 18 and solely within the area defined by the post-tensioned floor slab 20, as shown in FIG. 3C. The tendons 22 may be placed upon chairs 52 to vary the height of the tendons 22 within the post-tensioned, concrete floor slab 20 and the beams 18. Locations and heights of the various chairs 52 are specified according to engineering requirements.

[0043] The ends of each tendon 22 differ as depicted in FIG. 2 and in greater detail in FIGS. 3A and 3B. A first end 28 of a tendon 22 is a dead end anchorage 26. This first end 28 is embedded in the concrete post-tensioned floor slab 20 or concrete beam 18 at a first side or end 38 of the post-tensioned floor slab 20 or beam 18 and does not move once the concrete 40 cures. The second end 32 of the tendon 22 is similarly embedded within and near the second side or end 48 of the concrete post-tensioned floor slab 20 or the concrete beam 18. The second end 32 of the tendon 22 is an active end wherein the tendon 22 protrudes from the concrete post-tensioned floor slab 20 or beam 18, which allows a force to be applied to the tendon 22 and place it under tension during the building process and permanently thereafter.

[0044] Also depicted in FIG. 2., the exterior foundation walls 54 (shown as solid lines) of the basement are formed directly on top of at least a portion of the top surface perimeter 56 of the concrete floor slab 20. FIG. 4A shows a cross section of the intersection of a foundation wall 54 and the post-tensioned floor slab 20 in greater detail. Reinforcement members 58, for example, rebar or other mild steel reinforcement, may be placed within the concrete 40 to strengthen the foundation walls 54 and tie the foundation walls 54 to the post-tensioned floor slab 20. An angled reinforcement member 59 may also be placed to vertically support the position of a tendon 60 in the foundation wall 54. A sill 61 may be mounted to the top of the foundation walls 54 by anchor bolts 57 embedded within the foundation wall 54. The sill 61 provides the base for framing the superstructure of the house or other structure to be built upon the foundation 17.

[0045] In addition to exterior foundation walls 54 of the basement, interior foundation walls 55, as shown in FIG. 2, may similarly be formed upon the post-tensioned, concrete floor slab 20. Such interior basement walls 55 may perform several functions, for example, support a point load from the superstructure above, act as a beam to support the superstructure above, support the ends of beams spanning from the exterior foundation walls 54, and acting as a buttress to exterior foundation walls 54 to counteract the lateral forces of the soil backfilled against the exterior foundation walls 54. Rather than full interior walls 55, interior concrete buttresses 57 may be formed on the post-tensioned floor slab 20 to oppose lateral soil forces on the exterior foundation walls 54. Alternatively, counterforts 170 external to the exterior foundation walls 54 may be provided to oppose the lateral soil forces on the exterior foundation walls 54. As shown in FIG. 2, the counterforts 164 may be constructed, in one embodiment, by forming vertical concrete members 165 on extensions 170 of the post-tensioned floor slab 20. The counterforts 164 may be structurally integrated into the post-tensioned floor slab 20 by extending a tendon 22 from the post-tensioned floor slab 20 into the extension 170 and the vertical concrete member 165. Various novel embodiments of counterforts 164 will be described later herein with respect to FIGS. 9A-11B.

[0046] Application of one aspect of the invention may be described generally with respect to a home constructed with a basement wherein the basement is excavated at a single level as shown in plan view in FIG. 2. In order to prepare a post-tensioned, concrete foundation system 17 below grade, at either single or multiple levels, according the present invention, the basement or other below grade levels must first be excavated at the home site. In some instances, an advantage of the present inventive foundation system 17 is that the excavation will generally be several feet less than the depth necessary to build an elevated structural floor. This is because the extra foundation height required for a structural floor is not necessary. The excavation will, however, need to be a few feet wider than the actual length and width of the foundation 17 on two adjacent sides of the excavation to allow access to the active ends of the tendons 22 to tension the concrete 40. Once the home site has been excavated, it may be desirable to additionally trench the perimeter area over which the foundation floor slab 20 is to be poured in order to pour structural concrete beams 18 under the perimeter 56 of the basement floor slab 20 as shown in FIG. 2. It may be further desirable to trench areas across the length and width of the floor slab 20 in order to pour beams 18 to add additional support to the floor slab 20. The beams 18 can be poured either monolithically with the floor slab 20 or separately as described below. The use of beams 18 may not be necessary as previously indicated depending upon the engineering design of the structure. In some instances, especially with zero property line build-outs in many residential neighborhoods today, it may be advantageous to excavate two or more home sites in a single pit, thereby overlapping the additional width needed to tension the tendons during the concrete cure.

[0047] Once the trenches for beams 18 have been excavated, tendons 22 for tensioning the beams may be placed. One method for post-tensioning concrete known in the art that may be used is the addition of a mono-strand tendon 22 with a dead-end anchorage 26 on a first end 28 and a stressing anchorage 30 on a second active end 32 to the beam 18. The monostrand tendon 22 may consist of a seven wire, steel strand 24 coated with a corrosion-inhibiting grease. The seven wire, steel strand 24 may be encased in an extruded plastic protective sheathing 34. An iron or steel plate 36 is attached to the first end 28 of the monostrand 24 normal to the length of the tendon 22 to provide the dead-end anchorage as shown in FIG. 3A. The dead-end anchor 26 will generally be placed a few inches from a first end 38 of the beam 18 and supported such that it is completely embedded in the concrete 40 when the concrete 40 is poured. On a second end 32 of the tendon 22, the stressing anchorage 30 is supplied as shown in FIG. 3B. The stressing anchorage 30 may be an iron or steel casting 42 that grips the mono-strand 24. The casting 42 is placed immediately behind a plastic grommet 44 through which the tendon 22 extends. The grommet 44 is placed against the concrete form 46 at the second end 48 of the beam 18 to create a pocket 50 around the tendon 22 protruding therefrom and preventing concrete 40, when poured, from contacting the exposed steel mono-strand 24 of the tendon 22.

[0048] The tendons 22 are placed along the length of the beam 18 by draping points of the tendon 22 over one or more support chairs 52 as shown in FIG. 3C. The support chairs 52 may be of varying heights and are placed according to engineering specifications to alter the vertical position of the tendon 22 through the concrete beam 18 in order to achieve the desired structural strengthening effect. Portions of the tendons 22 may be raised above the top of the beam 18 to extend within the area of the post-tensioned floor slab 20 to structurally integrate the two. Reinforcement members 58, e.g., mild steel rebar, may also be placed throughout the trenches in order to provide additional reinforcement for the beams 18 as desired. The beams 18 may be poured either monolithically or separately from the post-tensioned floor slab 20. If a beam 18 is poured separately from the pouring of the post-tensioned floor slab 20, additional reinforcement members 58 may be placed protruding above the beam 18 in order to tie the beam 18 into the post-tensioned floor slab 20 when the concrete for the post-tensioned floor slab 20 is poured.

[0049] Once tendons 22 have been placed for the beams 18, additional tendons are then placed for tensioning the floor slab 20 itself. Tendons 22 may be placed laterally and longitudinally across the area of the floor slab 20 according to desired engineering specifications. As used with the beams 18, each of the tendons 22 has a dead-end anchorage 26 on a first end 28 and a stressing anchorage 30 on an active second end 32. Preferably, when multiple tendons 22 are used, each of the dead-end anchorages 26 of adjacent, parallel tendons 22 are placed along the same perimeter edge 56 of the foundation structure 17. The tendons 22 are again draped along support chairs 52 of varying heights, with the height and spacing of the chairs 52 determined according to engineering specifications for the post-tensioned floor slab 20. Again, reinforcement members 58 may be placed throughout the area of the floor slab 20, for example, to reinforce corners, to reinforce cut-out areas within the floor slabs 20 (e.g., for placement of a sump-pump), and for tying foundation walls 54 into the post-tensioned floor slab 20. Once the tendons 22 and reinforcement members 58 are in place, a cement mixture can then be poured to form the concrete 40 of the post-tensioned floor slab 20. As previously indicated, the post-tensioned floor slab 20 may be poured monolithically with the beams 18 directly on floor of the basement excavation. When leveling and smoothing the post-tensioned floor slab 20 during the pour, it may be desirable to leave the perimeter edges 56 of the post-tensioned floor slab 20 rough to lessen slippage between the floor slab 20 and the foundation walls 54 when the foundation walls 54 are later poured.

[0050] After the walls 54 have cured for a sufficient time, for example, several days, the tendons 22 in the post-tensioned floor slab 20 and beams 18 may be tensioned by any methods known in the art. Although one method for tensioning the floor slab 20 and beams 18 is described herein, other methods may be used with the same result. In an exemplary method, each exposed steel mono-strand 24 is first inspected and marked to indicate its pre-tensioning position. The grommet 44 is removed and a conical, two- or three-piece wedge (not shown) is placed around the exposed steel mono-strand 44 in the recess in the concrete formed by the grommet 44. A hydraulic jack (not shown) is attached to the steel mono-strand 24 and tensions the tendon 22 until it stretches the desired engineered distance. The wedge around the steel mono-strand 24 is placed such that it prevents the tendon 22 from retracting back into the floor slab 20. The corrosion-resistant grease and protective sheathing 34 around the steel mono-strand 24 act to break the bond between the concrete 40 and the tendon 22 and reduce friction on the steel mono-strand 24 during the tensioning process. The wedge maintains the tension on the tendon 22 and helps transfer the tension to the surrounding concrete 40. After sufficient time, for example, several days, the exposed steel mono-strands 24 of the tendons 22 are again inspected to ensure that there has been no slippage of the tendon 22. If everything is in order, the steel mono-strand 24 is cut to not extend beyond the edge of the concrete floor slab 20 and the recess formed by the grommet 44 is grouted with a corrosion-resistant mortar to seal the end of the tendon 22. Because the tendons 22 are permanently stressed, a constant compressive force acts upon the concrete floor slab 20 and beams 18 of the foundation 17. This compression counteracts the tensile forces created by loads upon the concrete floor slab 20, for example, cars, furniture, and the weight of the structure of the house itself, and increases the bending or beam strength of the floor slab 20 and beams 18. By post-tensioning the concrete floor slab 20, the load carrying capacity of the floor slab 20 is significantly increased.

[0051] At a suitable time after the tendons 22 in the post-tensioned floor slab 20 and beams 18 have been tensioned, the foundation walls 54 may be formed and poured as shown in FIG. 4A. Forms (not shown) for the exterior foundation walls 54 of the basement are placed over the perimeter edge 56 of the post-tensioned floor slab 20 so that the resultant foundation walls are supported on the perimeter of the foundation. Any reinforcement members 58 or tensioning cables 60 for strengthening the walls 54 are then placed appropriately within the forms. A cement mixture is then poured within the forms to create the concrete foundation walls 54. The concrete foundation walls 54 are formed directly onto the rough perimeter edge 56 of the post-tensioned floor slab 20. As previously indicated, the rough interface between the perimeter edge 56 and the foundation walls 54 helps reduce any slippage of the foundation walls 54 on the post-tensioned floor slab 20. Although formed concrete foundation walls 54 are primarily described herein, other types of foundation walls, for example, concrete block, brick, rock, and wood frame, may all be formed on the below grade post-tensioned floor slab 20 of the present invention. Thus, the foundation walls 54 are supported by the post-tensioned floor slab 20, rather than footings or caissons, and help to exert opposing force to counteract expansive soil forces.

[0052] Alternately, as shown in FIG. 4B in a design with a long basement wall, portions of the tendon 22 running in the plane of an exterior foundation wall 54 may undulate between the beam 54, the post-tensioned floor slab 20, and the foundation wall 54. The tendon 22 may terminate at each end within the foundation wall 54, rather than in the beam 18 or post-tensioned floor slab 20. In this manner, the beam 18, post-tensioned floor slab 20, and foundation wall 152 can be tensioned and compressed together, whereby the foundation wall 54 now acts as part of the load supporting beam 18 and further integrates the structure to react unitarily in opposition to expansive soil forces. Actual tendon placement may depend upon various factors, e.g., the load to be carried, the design tolerances required, the expansiveness of underlying soil, and the geometry of the house design. In a foundation design with a thick post-tensioned floor slab and without beams as described earlier, the tendon may undulate and alternatively run within a length of the basement slab and within a length of the basement wall. By similarly integrating all of the foundation walls, beams, and slabs under compression through the use of tensioning tendons, a monocoque structure is created to resist the forces of expansive soil and help cause the house to act as a unit. With the unification of the foundation wall 54 and the post-tensioned floor slab 20, the function of the beam 18 is distributed upward into the foundation wall 54 above the post-tensioned floor slab 20, thereby increasing the load carrying capacity of the foundation 17, lessening the excavation depth required for the beam 18, or both, and helping resist the effect of expansive soil.

[0053] By constructing a post-tensioned concrete floor slab 20 in an excavation as a basement floor and forming the concrete foundation walls 54 of the basement directly on the post-tensioned floor slab 20, the detrimental effects of expansive soils on the house foundation may be mitigated. The use of this construction structure and technique also offers several advantages over prior art construction structures and techniques designed to circumvent expansive soil problems. First, as noted, the post-tensioned floor slab 20 has greater resistance to soil expansion because the weight of the entire house, including the concrete basement walls 54, rests upon the post-tensioned floor slab 20 rather than merely upon beams 18. Second, because the post-tensioned floor slab 20 is poured directly on soil there is no space underneath the basement floor. Therefore, there is greatly reduced and likely no void space between the post-tensioned floor slab 20 and the soil at the base of the excavation and thus reduced opportunity for mold growth or vermin infestation as in a raised structural floor. Third, pouring a concrete post-tensioned floor slab 20 may be less time-consuming and labor intensive than building a structural floor. Fourth, less concrete may be used in forming the foundation walls 54, as the foundation walls 54 of the present design may be several feet shorter in height than the walls required for structural floor construction. Fifth, there is potentially less excavation involved because the excavated level for pouring a post-tensioned floor slab 20 of the present invention is several feet above the level required for building a home with a structural floor.

[0054] The post-tensioned, below-grade foundation 17 of the present invention is designed to be an integral unit in order to mitigate the effects of expansive soil. Construction of an integral foundation 17 through post-tensioning of the basement floor slab 20, forming the foundation walls 54 directly upon the post-tensioned floor slab 20, and structurally tying the foundation walls 54 and the post-tensioned floor slab 20 together helps create a unitary structure. If the foundation 17 moves at all due to soil expansion, the foundation 17 will move as a unitary body, thus reducing the likelihood of cracking and buckling of the foundation 17 or causing stress to the superstructure of the house.

[0055] In homes constructed with a basement and an attached garage, integration of the entire foundation can be difficult. Given the significant weight of automobiles that are normally parked in an attached garage, the floor of the garage is generally a concrete slab poured on grade. The garage floor slab could be formed as a post-tensioned slab on grade to resist the expansive soil forces. However the grade for pouring a garage slab will generally be significantly higher than the base of the excavation on which the basement floor slab sits, as it is necessary for the garage to be at ground level to provide access for cars to and from the street. While the walls for an attached garage are generally integrated into the framed superstructure of the house, the slab for the garage may be poured independent of the rest of the foundation of the house. Therefore, in response to expanding soil conditions, the garage slab may move independent of the house foundation, which may result in structural problems in the structural frame at the juncture of the garage and the house in addition to any problems with the foundation.

[0056] As shown in FIGS. 5, 6A, and 6B, one solution to the problem of integrating the garage 66 and the house is to construct a high load bearing, structural garage floor 64 over the post-tensioned basement foundation 68 of the present invention. The basement foundation 68 in FIG. 5 may be the foundation 17 shown in and described with respect to FIG. 2. FIG. 5 depicts only the foundation walls, and does not depict the tendons-22 post-tensioning the concrete floor slab 20 and the beams 18 supporting the post-tensioned floor slab 20 as shown in FIG. 2. In this embodiment, the basement actually extends underneath the structural garage floor 64. In this manner, the weight of the garage and any vehicles within it will be supported by the post-tensioned basement floor slab 70 and any beams underneath, if any. In this embodiment the substantial excess weight attributable to the garage 66 and automobiles will enhance the counteractive force of the foundation 68 against the pressure of expanding soils. The front and outside superstructure walls (not shown) of the garage are supported by the exterior basement walls (76 and 78, respectively, in FIG. 5), which in turn sit upon the post-tensioned concrete slab basement floor 70. In the design shown in FIG. 5, the front portion 82 of the interior garage wall superstructure (not shown) is also supported by a portion of the exterior basement wall 80 formed upon the post-tensioned floor slab 70. The remainder of the structural garage floor 64 is supported by an interior basement wall 84 and an internal buttress 100, each formed upon the post-tensioned floor slab 70.

[0057] As shown in FIG. 6A, the structural garage floor 64 is supported by horizontal steel beams 86 that are attached at one end to the either the jutting exterior wall 80 or the interior basement wall 84 and at the opposite end to the exterior basement wall 78 by vertically oriented steel angle plates 88. These steel angle plates 88 are bolted into each of the concrete jutting exterior wall 80, the concrete interior basement wall 84, and the concrete exterior basement wall 78 with expansion bolts 90 and bolted to the steel beams 86 with thru-bolts 92. As shown in FIG. 6B, the front edge 94 of the structural garage floor 64 is supported partially by the front basement wall 76 itself and partially by horizontally oriented steel angle plates 96 that are also bolted to the front basement wall 76 with expansion bolts 90.

[0058] The rear 98 of the structural garage floor 64 may be partially supported by an internal buttress 100 (as shown in FIG. 5) extending into the basement from the outside basement wall 78. Steel angle plates 96 may likewise be used along the buttress 100 to support the rear 98 of the structural garage floor 64 in a similar manner as the steel angle plates 96 on the front basement wall 76 support the front edge 94 of the structural garage floor 64. The internal buttress 100 is poured on top of the post-tensioned basement floor slab 70. The interior buttress 100 need not extend the entire width of the garage 66 but may extend only a portion of the width of the garage 66. The remaining span between the buttress 100 and the interior basement wall 84 may be spanned by another steel beam 102. The steel beam 102 may rest in pockets 104 formed in the tops of the interior wall 84 and the buttress 100 as shown in FIG. 5.

[0059] Once the steel beams 86 are anchored in place, metal pans or decking 106, as shown in FIG. 6A and 6B, may be placed on top of the beams 86 and overlap the front wall 76 and rear buttress 100. Reinforcement members 108 may then be placed above the metal decking 106. A cement mixture may then be poured over the metal decking 106 and reinforcement members 108 to form a concrete garage floor slab 62 for the structural garage floor 64. If desired, tendons could likewise be used to reinforce the concrete garage floor slab 62. Upon tensioning such tendons, the garage floor slab 62 would likewise be a post-tensioned structure. The garage floor slab 62 may be poured to extend over the top of the front basement wall 76 in order to meet a concrete slab for the driveway, which is poured on grade.

[0060] The internal buttress 100 supporting the rear 98 of the garage 66 serves an additional function of providing support to the outside basement wall 78 to resist the horizontal forces of the soil backfilled against the foundation walls. Also, by using the interior buttress 100 in cooperation with the steel beam 102 at the rear of the garage 66, the size requirement for the steel beam 102, especially one in a load bearing position, is substantially reduced. Further, by using the combination of the steel beam 102 and internal buttress 100 at the rear 98 of the garage 66, rather than extending the buttress 100 as an internal concrete wall across the width of the structural garage floor 64, access is allowed to the space under the garage 66, creating a room of usable space with the post-tensioned basement slab 70 for a floor and the structural garage floor 64 for a ceiling.

[0061] An alternative configuration combining caissons and a post-tensioned structural floor is also contemplated. In this embodiment, caissons may be placed in the basement excavation according to design to support a structural floor. Rather than hanging steel beams from the foundation walls as in the garage discussed with respect to FIGS. 5, 6A, and 6B, the steel beams may be supported by the caissons. A structural concrete floor slab as described with respect to FIGS. 6A and 6B may then be supported on the caissons and steel beams. The structural concrete floor slab may include tendons for post-tensioning. Once the structural floor slab is post-tensioned, the foundation walls may be formed on the post-tensioned floor slab as described with respect to FIG. 2. In this embodiment, there may be a void space beneath the structural slab floor and the soil at the base of the basement excavation.

[0062] In a further embodiment of the invention, the integration of multiple foundational slabs constructed at multiple excavation levels or at grade may be achieved, for example, as shown in FIGS. 7, 8A, and 8B. In the plan view of the foundation 110 depicted in FIG. 7, a high load bearing garage floor slab 112 is constructed on grade, higher than the basement slab 114, but is formed as an integral part of the foundation 110 according to the present invention. Through the disclosed design, the entire foundation 110, including the garage floor slab 112, reacts in a unitary manner to the effects of soil expansion. While a post-tensioned garage floor slab 112 on grade is described herein as an exemplary embodiment of a multi-level foundation according to the present invention, those skilled in the art will recognize that structures and methodologies used to integrate a post-tensioned slab on grade are likewise applicable to the integration of a separate foundation levels each below grade.

[0063] As detailed in FIGS. 7, 8A, and 8B, a post-tensioned garage floor slab 112 is formed on grade with tensioning cables (116, 118) running longitudinally and laterally through the slab. The longitudinal tendons 116 running from the front to the rear of the garage floor slab 112 may be extended through the top of the front wall 120 of the basement 122. As shown in FIG. 5B, the post-tensioned garage floor slab 112 may be poured monolithically with a concrete beam 124 supporting the perimeter of the garage floor slab 112 and a concrete sill 128 rising above the top of the slab 112 and extending along each of the lateral sides (130, 132) of the garage 126 from the front 125 of the garage 126 to the rear 127 of the garage 126. A first tendon 134, as shown in FIG. 8A, may be run through the beam 124 along each of the lateral sides (130, 132) of the garage 126 from the front 125 of the garage 126 to the rear 127. As shown in FIG. 8A, the bottom projection of the beam 124 for the interior side 132 of the garage 126 may be sloped downward toward the rear 127 of the garage 126 to eventually meet with the front edge 136 of the post-tensioned basement floor slab 114.

[0064] On top of the post-tensioned basement floor slab 114 may be an interior buttress 138, shown in FIGS. 7 and 8A, formed on top of the basement floor slab 114 normal to the front wall 120 of the basement 122 and extending from the front wall 120 of the basement 122 into the interior of the basement 122. The buttress 138 may also function as an interior wall of the basement 122. The slope of the beam 124 from the garage 126 approaching the basement floor slab 114 may be about 30%, and could be more or less depending upon the application. The slope of the first tendon 134 between the garage beam 124 at grade 140 and the excavation level 142 of the post-tensioned basement floor slab 114 should be gradual rather than sharp in order to provide the strongest integration between the basement foundation 122 and the garage floor slab 112. The first tendon 134 in the garage beam 124 follows the downward slope of the beam 124 and may be set within the buttress 138 above and parallel to the post-tensioned basement floor slab 114. Alternatively, the a portion of the first tendon 134 may also extend into the basement floor slab 114, further structurally integrating the garage 126 and the basement foundation 122.

[0065] In a further embodiment, a concrete beam 139 may run underneath the basement floor slab 114 under the area of the buttress 138 and may also extend to the rear 158 of the basement foundation 122. The first tendon 134 may thereby be run below the basement floor slab 114 within the beam 139. Alternately, the first tendon 134 may undulate between the basement floor slab 114 and the beam 139 as called for by the engineering design. Further, the first tendon 134 may undulate between the basement floor slab 114, the beam 139, and the internal buttress 138. If, the internal buttress 138 were instead an interior concrete wall, a portion of the first tendon 134 could likewise extend within such interior wall as well. This concept is further described with respect to FIG. 12 herein. In general, using present tendon technology, a tendon in transition from one level of foundation to another should not be bent at smaller than a five-foot radius.

[0066] A second tendon 144 may run through the concrete sill 128 on the interior side 132 of the garage 126 as shown in FIGS. 8A and 8B. This second tendon 144 may extend through the sill 128 from the front 125 of the garage 126 to the rear 127 of the garage 126 and through the top of the interior buttress 138 formed on the basement floor slab 114. The second tendon 144 therefore remains generally level and does not angle downwardly like the first tendon 134 in the beam 124.

[0067] In an alternative embodiment, the garage 126 may be positioned further rearward with respect to the house, for example, as positioned in FIG. 5. In this embodiment, the garage slab 112 would again be formed on grade and there would not be a basement area underneath the garage slab 112 as there is under the structural garage floor 64 of FIG. 5. Recognizing the short length of the garage slab 112 extending in front of the house (again, contemplating the garage slab 112 as formed on grade in the area of the garage 126 of FIG. 5), there may not be enough distance to extend the first tendon 134 through a gradually sloping beam 139 to ultimately extend the first tendon 134 into the basement floor slab 114. An alternative solution presented by this embodiment is to use the interior wall 84 of FIG. 5 (which in this embodiment would actually be an exterior wall of the foundation) to transition the first tendon 134 from the garage slab 112 at grade to the level of the basement floor slab 114 at the bottom of the basement excavation. The second tendon 144 could extend through the sill 128 from the front 125 of the garage 126 to the rear 127 of the garage 126 and through the top of the interior wall 84.

[0068] Returning to the embodiment of FIG. 7, as shown in greater detail in FIG. 12, a third tendon 150 may be placed in the foundation along the exterior side 130 of the garage 126 to tie the garage 126 into the basement foundation 122 as depicted in FIG. 12. The third tendon 150 may be run through the beam 124 below the garage slab 112, which extends downward toward the rear 127 of the garage 126 at a gradual slope to meet with the post-tensioned basement floor slab 114. In this instance, rather than a buttress, the third tendon 150 is run within the length of the outside basement wall 152 to the rear 158 of the basement. In one embodiment (not shown), the third tendon 150 may remain within the outside basement wall 152 parallel to the surface of the basement floor slab 114. Alternatively, the third tendon 150 may be run within either basement floor slab 114 or the beam portion 154 under the perimeter of the basement floor slab 114, parallel to the surface 156 of the basement floor slab 114. Alternately, portions of the third tendon 150 may undulate between the beam 154, the basement floor slab 114, and the exterior basement wall 152. When the third tendon 150 approaches the rear 158 of the basement foundation 122, as shown in FIG. 12, the third tendon 150 may be angled upward, emerging out of the basement slab 114 to terminate within the outside basement wall 152 at the rear 158 of the foundation 122.

[0069] A fourth tendon 160 may be run through the concrete sill 124 of the exterior side 130 of the garage 126 at or just below the surface of the garage floor slab 112, extending from the front 125 of the garage 126, to the rear 127 of the garage 126, and beyond through the top of the exterior basement wall 152 to the rear 158 of the foundation 122. The fourth tendon 160 may remain generally level with the top of the garage floor slab 112 for its entire length. The sloping beam 124, the garage floor slab 112, the interior buttress 138 on the post-tensioned basement floor slab 114, and the front basement wall 120 and the exterior basement wall 152 may all be poured monolithically once all the tendons (116, 118, 134,144, 150, 160) are in place. Alternatively, these same elements may also be poured in stages, depending upon the practicalities of the design.

[0070] When the tendons are stretched and placed under tension, the garage floor slab 112 is then integrated with the post-tensioned basement floor slab 114 and the rest of the foundation 110 as shown in FIG. 7. The post-tensioned garage floor slab 112 may be viewed as being effectively cantilevered from the basement foundation 110. In this manner, the post-tensioned garage floor slab 112 and the post-tensioned basement floor slab 114 are structurally tied together and the basement foundation 110 and the garage 126 become a single, integral unit. Again, it should be apparent that these inventive techniques described with respect to constructing a garage may be used to structurally integrate different sections of foundation formed at different levels at or below grade to construct a multilevel home. Thus, an integrated, multi-level, post-tensioned basement slab foundation may be constructed, which in turn supports the foundation walls and the superstructure of the house.

[0071] In a further embodiment of the invention, as shown in FIGS. 9A and 9B, the post-tensioned basement floor slab 162 is designed to provide counterforts 164 to help resist the horizontal soil forces on the basement walls 166 inward. While interior buttresses as described above work well in opposing the horizontal forces of soil backfill against basement walls, such buttresses may not be preferred as they intrude upon usable space in the basement. In order to strengthen the basement walls 166, counterforts 164 on the exterior side of the basement walls 166 may be used. A new design for counterforts 164 is provided by the present invention. In this aspect of the invention, and extension 170 of the post-tensioned basement floor slab 162 projects beyond the perimeter 163 of the foundation in the desired location of a counterfort 164. Reinforcement members 168 may be placed to protrude from the post-tensioned basement slab extension 170 and tie into additional reinforcement members 168 protruding from the basement wall 166 as shown in FIG. 9A. The counterfort 164 may then be formed on top of the post-tensioned slab extension 170 in a first embodiment as a vertical concrete member 165 normal to the adjacent basement wall 166. However, the counterfort 164 may be formed in another orientation other than vertical. The counterfort 164 may be poured monolithically with or separately from the basement wall 166.

[0072] By integrating the counterfort 164 with the post-tensioned floor slab 162, greater resistance to fracturing of the counterfort 164 due to expanding soil as well as better counterfort anchoring may be provided. The weight of the soil on the post-tensioned slab extension 170 creates a downward force on the counterfort 164 opposing the horizontal force moment of the soil against the basement wall 166 because of the integral structure formed by the post-tensioned concrete slab extension 170 as the base of the counterfort 164. The post-tensioned concrete slab extension 170 may also be formed integrally with a beam (not shown in FIGS. 9A and 9B) supporting the post-tensioned concrete slab 162, which beam may be similarly be constructed to extend beyond the perimeter 163 of the foundation, similar to the post-tensioned slab extension 170 described above, and the counterfort 164 may then be constructed on top of the post-tensioned beam.

[0073] In an alternative embodiment, a counterfort 172 may be formed to further act as a window well 174 around a basement wall window opening 176 as shown in FIG. 10. In this embodiment, an extension 178 of the post-tensioned floor slab 162 may extend beyond the perimeter of the post-tensioned floor slab 162, which may have a width slightly larger than the width of the window opening 176. A window well 174 may be designed in any desired shape, e.g., rectangular, arcuate, or a combination, to best allow access to the window opening 176 or provide the required strength. An appropriately-shaped window well 174 may be constructed by forming a concrete counterfort 172 on top of the post-tensioned slab extension 178, to which the counterfort 172 may be integrated by the use of reinforcement members (not shown in FIG. 10) protruding from the post-tensioned basement slab extension 178. The counterfort 172 may be poured either separately from or monolithically with the basement wall 166 surrounding the window opening 176. The post-tensioned slab extension 178 would be previously formed integrally with the post-tensioned floor slab 162.

[0074] Alternatively, the counterfort 172 could be of pre-cast construction and placed upon the post-tensioned basement slab extension-178. In this arrangement, both the basement slab extension 178 and the basement wall 166 on either side of the window opening 176 may be provided with steel plates (not shown) embedded in the concrete. Opposing steel plates (not shown) may be formed within the pre-cast concrete counterfort 172 to abut the steel plates in the basement slab extension 178 and the basement wall 166 when the pre-cast counterfort 172 is set in place. The pre-cast counterfort 172 may then be affixed to a basement slab extension 178 and the basement wall 166 by welding or bolting each of the opposing steel plates together. Other suitable connection structures may likewise be utilized. By using a pre-cast counterfort 172, time and money can be saved in the field by dispensing with the need to build an appropriate form and casting the counterfort 172 in place. Economies of scale can be achieved through the mass production of such pre-cast counterforts 172 in a factory environment. However, by connecting the pre-cast counterforts 172 to a post-tensioned slab extension 178, the beam and bending strength of the post-tensioned floor slab 166 is imparted to the counterfort 172, providing improved strength in construction over the prior art.

[0075] In a further variation on the window well counterfort design, an opening 180 for a sump pump, for example, for use as part of a drainage system for the foundation, may be provided in the post-tensioned basement slab extension 178 as shown in FIGS. 11A and 11B. By combining the basement slab extension 178 with a window well-type counterfort 184, a sump pump may be placed outside the house within the opening 180 in the basement slab extension 178, while access to the sump pump is provided by the well 182 within the counterfort 184.

[0076] The post-tensioned concrete slab constructed below grade and supporting the entire mass of a residential dwelling, including foundation walls and the superstructure of the dwelling, has been presented herein as a novel construction alternative to resist the detrimental effects of expansive soils. Such post-tensioned basement foundations are quite versatile. The strength of such foundations allows for the support of heavy loads, for example, a garage, which further helps counteract the expansive soil forces. Post-tensioned foundations according to the present invention may further be composed of multiple levels that are all structurally integrated through the structures and techniques disclosed herein. Further, extensions of the post-tensioned slab concept allow for construction of additional reinforcing features, for example, integrated counterforts, that add to the stability and resistance of the foundation to the forces of expansive soils.

[0077] Further, by structurally integrating the foundation walls with the post-tensioned concrete slab formed below grade, and in some embodiments by additionally integrating multiple foundation levels, the entire structure may react to expansive soil forces in a more unitary manner. Because the structure moves as an integral unit, the soil expansion must be sufficient to move the weight of the entire house. As a result, the structure is more likely to remain unaffected by soil expansion forces that may have previously caused cracks to appear in the floor or in the foundation. If the force of the expansive soil overcomes the weight of the entire structure, then the structure will likely react as a single unit, and the foundation and may merely shift slightly rather than causing structural damage.

[0078] Although various embodiments of this invention have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention. It is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative only of particular embodiments and not limiting. Changes in detail or structure may be made without departing from the basic elements of the invention as defined in the following claims. 

What is claimed is:
 1. A structure with a basement comprising: at least one first post-tensioned concrete slab below grade; and at least one exterior concrete basement wall supported on a perimeter edge of said first post-tensioned concrete slab.
 2. The structure of claim 1 further comprising: a plurality of beams; and a high load bearing floor supported by said plurality of beams spanning a first portion of said exterior concrete basement wall and a second portion of said exterior concrete basement wall.
 3. The structure of claim 1 further comprising a plurality of beams; and an interior concrete basement wall; wherein said high load bearing floor is supported by said plurality of beams spanning said interior concrete basement wall and said exterior concrete basement wall.
 4. The structure of claim 3 further comprising at least one interior concrete buttress, wherein at least one of said plurality of beams spans said at least one interior concrete buttress and said interior concrete basement wall.
 5. The structure of claim 2 wherein said high load bearing floor is a garage floor.
 6. The structure of claim 1 further comprising: a concrete sub-beam extending underneath and supporting said first post-tensioned concrete slab; and at least one tendon; and wherein at least a first portion of said at least one tendon extends within said first post-tensioned concrete slab, and at least a second portion of said at least one tendon extends within said concrete sub-beam.
 7. The structure of claim 1 further comprising: a second post-tensioned concrete slab adjacent said exterior concrete basement wall; and at least one tendon; and wherein at least a first portion of said at least one tendon extends within said second post-tensioned concrete slab, and at least a second portion of at least one tendon extends within said exterior concrete basement wall.
 8. The structure of claim 7 wherein at least a third portion of said at least one tendon extends within said first post-tensioned concrete slab.
 9. The structure of claim 7 further comprising a concrete sub-beam extending underneath and supporting said first post-tensioned concrete slab, wherein at least a third portion of said at least one tendon extends within said first concrete sub-beam.
 10. The structure of claim 9 wherein at least a fourth portion of said at least one tendon extends within said concrete sub-beam.
 11. The structure of claim 7 wherein said second post-tensioned concrete slab is a garage floor.
 12. The structure of claim 1 wherein said at least one exterior concrete basement wall is post-tensioned.
 13. The structure of claim 1 further comprising: a second post-tensioned concrete slab adjacent said exterior concrete basement wall; an interior concrete basement wall; at least one tendon; and wherein at least a first portion of said at least one tendon extends within said second post-tensioned concrete slab, at least a second portion of said at least one tendon extends within said exterior concrete basement wall, and at least a third portion of said at least one tendon extends within said interior concrete basement wall.
 14. The structure of claim 13 wherein at least a fourth portion of said at least one tendon extends within said first post-tensioned concrete slab.
 15. The structure of claim 13 wherein said second post-tensioned concrete slab is a garage floor.
 16. The structure of claim 1 further comprising at least one tendon, wherein at least a first portion of said at least one tendon extends within said first post-tensioned concrete slab and at least a second portion of said at least one tendon extends within said exterior concrete basement wall.
 17. The structure of claim 1 further comprising: an extension of first said post-tensioned concrete slab beyond said exterior concrete basement wall; and at least one counterfort defined by a concrete member formed upon and generally normal to said extension of said first post-tensioned concrete slab.
 18. The structure of claim 17 wherein a portion of a tendon from said first post-tensioned concrete slab extends within said at least one counterfort.
 19. The structure of claim 17 further comprising a window opening located in said exterior concrete basement wall, and wherein said at least one counterfort defines a window well around said window opening.
 20. The structure of claim 17 further comprising a sump pump opening in said extension of said first post-tensioned concrete slab, wherein said at least one counterfort defines a sump pump well around said sump pump opening.
 21. The structure of claim 1 wherein said first post-tensioned concrete slab is supported by at least one post-tensioned concrete beam.
 22. A method of constructing a foundation resistive to effects of expansive soil, the foundation having a first concrete slab and an exterior concrete wall, the method comprising: excavating a below grade level in soil; placing a plurality of tendons in said below grade level; pouring a cement mixture over said plurality of tendons in said below grade level; at least partially curing said cement mixture to form said first concrete slab; post-tensioning said concrete slab by applying tensile forces to said plurality of tendons; and forming said exterior concrete wall on a perimeter of said first concrete slab.
 23. The method of claim 22 wherein the step of placing further comprises placing at least one of said plurality of tendons such that a portion of said at least one of said plurality of tendons extends within said exterior concrete wall.
 24. The method of claim 22 further comprising the step of supporting a high load bearing floor by said foundation.
 25. The method of claim 24 further comprising the step of hanging a plurality of beams between a first portion of said exterior concrete wall and a second portion of said exterior concrete wall; and constructing said high load bearing floor on said plurality of beams.
 26. The method of claim 25 wherein the step of forming further comprises forming an interior concrete wall; and he step of hanging further comprises hanging at least one of said plurality of beams between said exterior concrete wall and said interior concrete wall.
 27. The method of claim 26 further comprising forming an interior concrete buttress, and wherein the step of hanging further comprises hanging at least one of said plurality of beams between said interior concrete buttress and said interior concrete wall.
 28. The method of claim 24 wherein said high load bearing floor is a garage floor.
 29. The method of claim 22 further comprising the step of structurally integrating a second concrete slab with said foundation.
 30. The method of claim 29 wherein said second concrete slab is adjacent to said exterior concrete wall, and wherein the step of structurally integrating said second concrete slab to said foundation further comprises connecting said second concrete slab with said exterior concrete wall with at least one tendon.
 31. The method of claim 30 wherein the step of connecting further comprises connecting said exterior concrete wall and said first concrete slab with said at least one tendon.
 32. The method of claim 30 further comprising forming an interior concrete buttress, and wherein the step of connecting further comprises connecting said interior concrete buttress and said exterior concrete wall with said at least one tendon.
 33. The method of claim 31 wherein the step of connecting further comprises connecting said interior concrete buttress and said first concrete slab with said at least one tendon.
 34. The method of claim 29 wherein said second concrete slab is a garage floor.
 35. A below grade foundation for a structure comprising: a concrete slab; an exterior concrete basement wall on a perimeter edge of said concrete slab; and a means for post-tensioning said concrete slab.
 36. The below grade foundation of claim 35 further comprising a means for structurally integrating said concrete slab with said exterior concrete basement wall.
 37. A multi-level foundation for a structure comprising: a first level further comprising a first concrete slab, an exterior concrete basement wall on a perimeter edge of said first concrete slab, and a means for post-tensioning said first concrete slab; at least one additional level further comprising a second concrete slab; a means for structurally integrating said first level with said at least one additional level to form a unitary structure; and wherein at least one of said levels is below grade.
 38. The multi-level foundation of claim 37 wherein said at least one additional level supports a garage.
 39. A structure comprising: a below-grade concrete foundation; and a means for post-tensioning said foundation; wherein effects of expansive soil on said foundation are mitigated by said foundation by a combined force of said structure and said below-grade foundation counter to the expansive soil.
 40. A foundation system for a structure comprising: a post-tensioned concrete slab seated directly on soil below grade; an exterior concrete foundation wall positioned on a perimeter edge of said post-tensioned concrete slab; and wherein said foundation system counteracts forces created by expansion of the soil.
 41. The foundation system of claim 40 further comprising at least one post-tensioning cable, wherein at least a first portion of said at least one post-tensioning cable extends within said exterior concrete foundation wall and at least a second portion of said at least one post-tensioning cable extends within said post-tensioned concrete slab.
 42. The foundation system of claim 40 further comprising at least one internal concrete buttress generally normal to said exterior concrete foundation wall.
 43. The foundation system of claim 40 further comprising: an extension of said post-tensioned concrete slab; and at least one counterfort defined by a concrete member formed upon and generally normal to said extension of said post-tensioned concrete slab.
 44. The foundation system of claim 43 further comprising a window opening located in said exterior concrete basement wall, wherein said counterfort defines a window well around said window opening.
 45. The foundation system of claim 43 further comprising a sump pump opening in said extension of said post-tensioned concrete slab, wherein said counterfort defines a sump pump well around said sump pump opening.
 46. The foundation system of claim 40 further comprising at least a second concrete slab structurally integrated with said post-tensioned concrete slab.
 47. A counterfort comprising: an extension of a post-tensioned concrete slab beyond a wall of a foundation supported by said post-tensioned concrete slab, wherein said extension of said post-tensioned concrete slab is poured directly on soil of a basement excavation; and a concrete member formed upon and generally normal to said extension of said post-tensioned concrete slab.
 48. The counterfort of claim 47, further comprising an opening in said extension of said post-tensioned slab for housing a sump pump.
 49. The counterfort of claim 47, wherein said concrete member forms a window well.
 50. The counterfort of claim 49, further comprising an opening in said extension of said post-tensioned slab for housing a sump pump.
 51. A foundation beam for a structure comprising: a concrete sub-beam; a concrete slab formed upon said concrete sub-beam; a concrete wall formed upon said concrete slab; and a tendon of which a first portion extends within said concrete sub-beam, a second portion extends within said concrete slab, and a third portion extends within said concrete wall; wherein when tensioned, said tendon structurally integrates said concrete sub-beam, said concrete slab, and said concrete wall into said foundation beam.
 52. The foundation beam of claim 51, wherein said concrete slab is post-tensioned.
 53. The foundation beam of claim 51, wherein said concrete sub-beam, said concrete slab, and at least a portion of said concrete wall all formed below grade. 